This application is based on Japanese Patent Application No. 11-122749 filed on Apr. 28, 1999 and No. 2000-68928 filed on Mar. 13, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to the compression and expansion process of an image data using the discrete cosine transform technique, in particular, relative to the reduction of the mosquito noise.
2. Description of Related Art
Apparatuses such as copying machines, scanners and printers are capable of compressing and expanding an image data as well as removing under color from the image data.
The compression and expansion process is used for reducing the capacity requirement of a memory where the image data is stored and the JPEG (joint photographic experts group) method based on the discrete cosine transform is widely used. However, the compression and expansion process using the discrete cosine transform tends to cause the mosquito noise in the vicinity of an edge, where the density value of the pixel varies sharply, to cause deterioration of the picture quality. Incidentally, publication of Unexamined Japanese Patent Application, JP-A-5-294018 discloses a technique of removing the mosquito noise by means of correcting the data of the pixels in the vicinity of an edge using a filtering process.
On the other hand, the under color removal process is normally applied to images contained in printed matters such as newspapers and magazines, for example, images consisting of a nonwhite background and black characters. Specifically, the density value below a specified value is changed to xe2x80x9c0xe2x80x9d and the color of the background is changed to white. It is possible to allocate the maximum gradation to the necessary image data. However, the under color removal process increases the density differences among the image and the pixels in the vicinity of an edge.
Thus, the under color removal process is beneficial for securing the gradation but increases the chance of the mosquito noise generation during the compression and expansion process.
It is an object of the present invention to an apparatus for processing images including a compression-expansion unit, a first under color removing unit, and a second under color removing unit. The compression-expansion unit executes a compression and expansion process on an image data using discrete cosine transform. The first under color removing unit applies an under color removal process to the image data before the compression and expansion process. The second under color removing unit applies an under color removal process to the image data after the compression and expansion process.
A further object of the invention is to provide a method for processing images including the steps of executing a compression and expansion process on an image data using discrete cosine transform, applying an under color removal process to the image data before the compression and expansion process, and applying an under color removal process to the image data after the compression and expansion process.
Still a further object of the invention is to provide a computer program product for processing images including the steps of executing a compression and expansion process on an image data using discrete cosine transform, applying an under color removal process to the image data before the compression and expansion process, and applying an under color removal process to the image data after the compression and expansion process.